The present invention relates to a microactuator and a method of forming the same, and more particularly to a micro-actuator suitable for driving a small size device such as optical devices, optomagnetic and magnetic devices.
A convention small actuator is mounted on a top of a suspension of a magnetic head for driving a slider. This was proposed by L. S. Fan and is disclosed in IEEE Transactions On Industrial Electronics, vol. 42, No. 3, pp. 222-233, June 1995 entitled "Magnetic Recording Head Positioning At Very High Track Densities Using A Microactuator Based Two-Stage Servo System". FIG. 1 is a plane view illustrative of a conventional microactuator. FIG. 2 is a partially enlarged view of an area "A" in FIG. 1 illustrative of the conventional microactuator. FIG. 3 is a cross sectional elevation view illustrative of the conventional microactuator taken along an A--A line in FIG. 2. In FIGS. 1 and 2, illustrations of a structure of a platform are eliminated. The microactuator has a pair of stators 83 and 84 provided on opposite sides of a silicon substrate 100 and separated from each other in a first direction, and a movable part 82 positioned between the stators 83 and 84. The movable part 82 is supported by spring members 81 which are provided on spring fixing stages 80 fixed to the silicon substrate 100 so that he movable part 82 is floated or isolated from the silicon substrate 100.
Each of the stators 83 and 84 has a stator extending portion which extends toward inside regions and in the first direction and also extends along a longitudinal center line. The stator extending portion has many stator comb-tooth portions 91 which extend from both sides of each of the stator extending portions in a second direction perpendicular to the first direction, thereby to form a comb-shape, wherein the stator comb-tooth portions 91 are arranged at a first constant pitch in the first direction and extend in the second direction. Each of the stator comb-tooth portions 91 has comb-tooth shaped stator electrodes 93 which extend from one side of the stator comb-tooth portion 91, wherein the comb-tooth shaped stator electrodes 93 extend in the first direction at a second constant pitch.
Each of the movable part 2 comprises first and second side frame portions extending in the first direction and separated from each other in the second direction and a center frame portion extending in the second direction to connect the first and second side frame portions to each other. Each of the first and second side frame portions has many movable comb-tooth portions 92 which extend from the side toward the longitudinal center line in the second direction, thereby to form a comb-shape, wherein the movable comb-tooth portions 92 are arranged at a third constant pitch in the second direction and extend in the first direction. The many movable comb-tooth portions 92 and the many stator comb-tooth portions 91 are alternately aligned in the first direction, whereby each of the many movable comb-tooth portions 92 is positioned between adjacent two of the many stator comb-tooth portions 91. Each of the movable comb-tooth portions 92 has comb-tooth shaped movable electrodes 94 which extend from one side of the movable comb-tooth portion 92, wherein the comb-tooth shaped movable electrodes 94 extend in the first direction at a fourth constant pitch, so that the comb-tooth shaped movable electrodes 94 and the comb-tooth shaped stator electrodes 93 are alternately aligned in the second direction, whereby each of the comb-tooth shaped movable electrodes 94 is positioned between adjacent two of the comb-tooth shaped stator electrodes 93. The stator comb-tooth portion 91 is wider in width than the movable comb-tooth portion 92. The comb-tooth shaped stator electrodes 93 are wider in width than the comb-tooth shaped movable electrodes 94. The comb-tooth shaped stator electrodes 93 are adhered onto the silicon substrate 100 together with the many stator comb-tooth portions 91. The comb-tooth shaped movable electrodes 94 are separated or floated from the silicon substrate 100 together with the movable comb-tooth portion 92.
A voltage is applied across the comb-tooth shaped movable electrodes 94 and the comb-tooth shaped stator electrodes 93 so that the movable part 82 is driven to be moved in the first direction. The voltage application across the comb-tooth shaped movable electrodes 94 of the movable part 82 and the comb-tooth shaped stator electrodes 93 of the second stator 84 causes the movable part 82 to move toward the second stator 84. The voltage application across the comb-tooth shaped movable electrodes 94 of the movable part 82 and the comb-tooth shaped stator electrodes 93 of the first stator 83 causes the movable part 82 to move toward the first stator 83.
The above conventional microactuator has been fabricated a follows. A phospho silicate pattern of 2 micrometers in thickness if formed on a first region of the silicon substrate 100, wherein the first region is for later formation of the above movable part 82. A photo-resist pattern is then formed on the phospho silicate glass pattern by use of a photo-lithography technique. A copper plating method is carried out to form copper films between apertures of the photo-resist pattern. A platform pattern is then formed by use of other photo-lithography technique and subsequent copper plating method before the phospho silicate glass pattern is removed by an etchant of hydrofluoric acid solution so as to isolate the movable part 82 and the comb-tooth shaped movable electrodes 94 from the silicon substrate 100, whereby the microactuator and the platform are formed. Namely, this actuator is formed by the electro-plating technique.
By the way, it has been known in the part that a polysilicon thin film is used for the microactuator fabricated by utilizing a silicon IC process. This microactuator having the polysilicon thin film is superior in conformability with the silicon IC process and also in mechanical characteristics as compared to the above electroplate microactuator. In order to apply this second microactuator having the polysilicon film to the magnetic had or the optomagnetic head, it is, however, necessary to suppress motion of the head in other directions to the intended direction. For example, the microactuator shown in FIG. 1 is required to cause the movable part 82 to move in the first direction but required to suppress any motion of the movable part 82 in a vertical direction to the first and second directions. In order to suppress the motion of the movable part 82 in the vertical direction, it is effective to increase the thickness of the springs 81. In view of utilizing a large electrostatic force, it is also important to increase the thicknesses of the comb-tooth shaped movable electrodes 94 and the comb-tooth shaped stator electrodes 93.
Accordingly, it had been required to form the actuator having a thickness of not less than 20 micrometers. Notwithstanding, a practically possible maximum thickness of the polysilicon film is about 4 micrometers which is much thinner than the required thickness. For those reasons, the above plating technique for forming the electro-plated actuator and the following other type microactuator had been developed. The other type microactuator may be formed by etching technique for etching a single crystal silicon layer.
The actuator having the silicon crystal silicon film may be formed by use of an silicon-on-insulator substrate. This technique was proposed by A. Benitez et al. and is disclosed in Sensors and Actuators, A50, pp. 99-103, 1995, entitled "bulk silicon microelectromechanical devices fabricated from commercial bonded and etched-back silicon-on-insulator substrates". The use of this technique allows the comb-tooth shaped movable electrodes 94 and the comb-tooth shaped stator electrodes 93 to be formed from a single crystal silicon layer of 20 micrometers in thickness. The comb-tooth shaped stator electrodes 93 and the many stator comb-tooth portions 91 are fixed through a silicon oxide layer to the silicon substrate 100. The silicon oxide film underlying the comb-tooth shaped movable electrodes 94 is removed by a hydrofluoric acid solution as an etchant, so that the comb-tooth shaped movable electrodes 94 is made floated or isolated from the silicon substrate 100. Since the comb-tooth shaped movable electrodes 94 is narrower in width than the comb-tooth shaped stator electrodes 93 and also the movable comb-tooth portions 92 is narrower in width than the stator comb-tooth portions 91, and silicon oxide film remains under the comb-tooth shaped stator electrodes 93 and the stator comb-tooth portions 91 even after the wet etching has been carried out by use of the hydrofluoric acid solution. The comb-tooth shaped movable electrodes 94 having the single crystal silicon film and the comb-tooth shaped stator electrodes 93 also having the single crystal silicon film may be formed over the silicon substrate 100.
The comb-tooth shaped movable electrodes 94 may accidentally be made into contact with the silicon substrate 100 during the operation of the microactuator. If the comb-tooth shaped movable electrodes 94 is hit to the silicon substrate 100, the comb-tooth shaped movable electrodes 94 might be adhered to the silicon substrate 100, whereby the movable part 82 becomes inoperable. In the isolation process or isolating the comb-tooth shaped movable electrodes 94 from the silicon substrate 100, a water used for cleaning process is removed by dry process, whereby the surface tension of water might cause the comb-tooth shaped movable electrodes 94 to be adhered to the silicon substrate 100. This adhesion problem is particularly remakable when the single crystal silicon is used for the microactuator material because the image-force is applied between the comb-tooth shaped movable electrodes 94 and the silicon substrate 100. This adhesion problem is also risen when the microactuator is fabricated by the copper-electroplating method.
A method of solving the above adhesion problem was proposed by T. Yee et al. and is disclosed in Digest of the 8.sup.th international conference on solid-state sensors and actuators, vol. 1, pp. 206-209, June 1995, entitled "polysilicon surface modification technique to reduce sticking of microstructures".
This method will be described as follows. FIGS. 4A through 4E are fragmentary cross sectional elevation views illustrative of the second conventional microactuator in sequential processes involved in a conventional fabrication method thereof.
With reference to FIG. 4A, a silicon oxide film 121 is deposited on a silicon substrate 120 before a polysilicon film 122 having a thickness of 0.5 micrometers is deposited on the silicon oxide film 121.
With reference to FIG. 4B, a phospho-silicate glass film 123 is formed on the polysilicon film 122 whereby oxidation is deeply progressed into deep portions of grain boundary regions of the polysilicon film.
With reference to FIG. 4C, this sample is subjected to a mask-free dry etching process to increase difference in thickness of the silicon oxide film 123, thereby forming a polysilicon film 124 having an uneven surface.
With reference to FIG. 4D, a phospho-silicate glass film 126 having thickness of 2 micrometers is deposited on the polysilicon film 124. A polysilicon film 125 having a thickness of 2 micrometers is deposited on the phospho-silicate glass film 126. The polysilicon film 125 is patterned to be shaped into an actuator.
With reference to FIG. 4E, the phospho-silicate glass film 126 is removed by a wet etching using a hydrofluoric acid solution.
The contact surface is intended to be made rough so as to reduce a contact area between the movable electrode and the substrate, whereby an attractive force of a solid surface is reduced. The method of forming uneven contact surface to reduce the friction has been known. In Japanese laid-open patent publication No. 8-23685, it is disclosed that silicon oxide projections are provided between the movable part and the substrate. Those projections are formed by photo-lithography techniques so that plane size of each projection is much larger than the projections made in the above process in FIG. 8C. The friction and adhesion force largely depend upon the roughness of the contact surface, for which reason the actuator fabricated by the processes of FIGS. 8A through 8E is much superior than the other conventional actuator. The process of FIG. 8C for forming the rough surface is free from any photo-lithography. This is advantageous in simplicity of the fabrication process for the microactuator. The above processes of FIGS. 8A through 8E are inapplicable to the single crystal silicon microactuator using the silicon-on-insulator wafer. In accordance with the above processes of FIGS. 8A through 8E, the rough surface has been formed on the polysilicon film 122 on the silicon oxide film 121 on the silicon substrate 100, before the polysilicon film 125 is formed to be patterned into ether the movable part or stator.
The silicon-on-insulator substrate may be commercially available from the makers to reduce the cost and secure the high quality. The film, which should have to be made into movable part or stator, has already been deposited, for which reason it is impossible to make rough the surface of the substrate by use of the conventional method of FIG. 8C.
The other conventional microactuator have been known. In Japanese laid-open patent publication No. 6-53206, it is disclosed to form an operational gap in a silicon oxide film over a substrate by selectively removing the silicon oxide film.
In the above circumstances, it had been required to develop a novel microactuator and a method of forming the same free from the above problems.